The present invention generally relates to the devices for growing semiconductor layers, and in particular to a MOCVD device for growing a semiconductor layer on a substrate by the metal-organic vapor deposition process.
The metal-organic vapor deposition process referred to hereinafter as MOCVD process is one of the major processes for growing compound semiconductor layers having a complex composition on a substrate. Such a process is essential for the fabrication of super-fast semiconductor devices that utilize the group III-V compound semiconductor materials.
In the MOCVD process, the deposition of the semiconductor layer is achieved by the thermal decomposition of an organic or hydride source gas that contains the group III element such as aluminum, gallium, indium and the like, and the group V elements such as arsenic, phosphide and the like. Thereby, the source gas is introduced into a reaction chamber in which a semiconductor substrate is held, and the gas thus introduced is subjected to the thermal decomposition process that may be achieved either by a resistance heating fixture, induction heating fixture, or a radiation heating fixture that uses high-power lamps. As a result of the decomposition, the group III and the group V elements in the gases are released and deposited on the substrate in the form of the desired group III-V compound semiconductor layer.
In such a MOCVD process, the properties of the compound semiconductor layer changes significantly depending on the temperature on the substrate. For example, when a ternary or quarternary compound semiconductor layer is to be grown with the controlled property such as stoichiometry, composition or thickness, it is necessary to maintain the temperature of growth of the semiconductor layer as uniform as possible. Otherwise, the property of the obtained semiconductor layer may change place by place on the surface of the substrate. Even in the case of growing a simple binary compound semiconductor layer such as GaAs, the carrier concentration level of the obtained semiconductor layer may change depending on the place on the substrate surface when the semiconductor layer is doped and there is a temperature inhomogeneity in the growth of the compound semiconductor layer along the surface of the substrate. It should be noted that, even when the dopant elements are introduced with a predetermined concentration, the activation thereof achieved by incorporating the dopants into the predetermined crystallographic sites of the deposited compound semiconductor material, is affected strongly by the temperature in which the growth was made.
In order to control the property of the compound semiconductor layers grown by the MOCVD process, it is necessary to control the temperature of the substrate itself and the gas temperature, rather than the temperature of the susceptor that holds the substrate, as uniform as possible. This means that it is necessary to control the temperature of the gas that makes a contact with the substrate as uniform as possible. It should be noted that when the gas is first introduced into the reaction chamber, the gas generally has a low temperature such as the room temperature and is heated as it flows through the reactor. Thus, the uniform heating of the susceptor or holder of the substrate is not sufficient for obtaining the uniform quality in the grown compound semiconductor layer. In order to achieve the desired uniform quality, the capability of a complex control of the temperature is required for the heating fixture.
The radiation heating fixture utilizing the high-power lamps is advantageous from this view point, as the heating fixture can easily provide a controlled temperature profile by utilizing a number of lamps and controlling each lamp separately. On the other hand, the conventional resistance heating fixture or induction heating fixture lacks the flexibility of adapting to the different temperature profile, gas flow rate, gas composition, and the like.
FIG. 1 shows a prior art MOCVD device that uses a number of lamps for the heating. This device is the one disclosed by Frijlink (Frijlink, P. M., "A New Versatile, Large Size MOVPE Reactor," J. Crystal Growth vol.93, pp.207-215, 1988), which is incorporated herein as reference.
Referring to FIG. 1, the device comprises a reactor 10 having a reaction chamber 10a in which a gas or gas mixture is introduced via a tube 11. The tube 11 has a cone-shaped nozzle 11a for distributing the gas into the reaction chamber 10a, and the reaction chamber 10a has a circular shape in the plan view such that the gas introduced via the nozzle 11a flows along a radial path in the horizontal direction. After the thermal decomposition, the gas is evacuated through exhaust outlets 12 provided at the side wall of the reaction chamber 10a. In the reaction chamber 10a, there is provided a disc-like platform 13 that revolves about an axis substantially coincident to the tube 11, and a number of substrates 14 are provided on the platform 13 such that each substrate 14 revolves about respective rotational axes. In order to cause the thermal decomposition of the gas, a number of bar-shaped tungsten lamps 15 are provided under the platform 13 to extend parallel with each other. It should be noted that the bar-shaped tungsten lamps extend vertically to the sheet of drawing in FIG. 1. In operation, the bar-shaped tungsten lamps 15 are energized independently. Thereby, a uniform temperature distribution is achieved in the reaction chamber.
Such a fixture, however, has a problem in that the temperature profile in the elongating direction of the bar-shaped lamps 15 is not controlled. In other words, the device of FIG. 1 does not provide a uniform temperature distribution in the gas flow in the reaction chamber even when the lamps are controlled individually. Thereby, a complex fixture has to be used for revolving each substrate about their axes while simultaneously revolving the platform 13 about the central axis to homogenize the temperature on the substrate. Further, the MOCVD device of FIG. 1 has a problem in that the size of the device tends to become excessively large because of the use of the disc-shaped platform that requires a lateral placement of the substrate 14 thereon. Such a construction raises a problem particularly when a number of substrates 14 are to be placed on the platform 13 or the size of the substrate 14 is increased. In such a case, it is inevitable to increase the diameter of the platform 13. However, such an increase of platform diameter causes a problem of excessive size of the MOCVD device.
FIG. 2 shows another prior art MOCVD device that uses the lamp heating fixture. The device is the one disclosed in the Japanese Laid-open Patent Application No. 59-36927, which is incorporated herein as reference.
Referring to FIG. 2, the MOCVD device has a bell-jar vessel having a base 21 and an outer, bell-shaped wall 22. Inside the outer wall 22, there is formed a corresponding inner wall 23 with a space defined therebetween, and a number of ring-shaped tungsten lamps 29 are accommodated in the space. It should be noted that each of the ring-shaped lamp 29 defines a plane extending substantially in the horizontal direction. The inner wall 23 defines a reaction chamber 23a in which a gas is introduced via a tube 21 that penetrates through the base 21. The gas thus introduced are evacuated, after the thermal decomposition, via an exhaust outlet tube 25. Further, a rotary shaft 26 extends into the reaction chamber 23a via the base 21 and a disc-shaped platform 27 is provided on the tip end of the shaft 26. Further, a substrate 28 is held on the platform 27 for the growth of the compound semiconductor layer thereon.
In operation, the source gas of reactants is introduced into the reaction chamber via the tube 24 and the tungsten lamps 29 are energized. Thereby, each lamp 29 is controlled such that the output power is large in the lamps located radially outer part while the output power is small in the lamps at the radially inner part in order to compensate the difference in the heat dissipation rate in the substrate. Thereby, a uniform heating of the substrate is achieved.
In this conventional device, however, the control of the gas temperature for realizing the uniform decomposition temperature and hence the uniform crystal growth, is not made. The cold gas introduced in the bell-jar vessel 20 flows along a complex pattern in the reaction chamber 23a and thus, it is extremely difficult to achieve the uniform gas temperature on the surface of the substrate 28 by the control of the tungsten lamps 29. Further, the device of FIG. 2 has a problem similar to the problem of FIG. 1 in that, when a large number of substrates are used for the growth of the compound semiconductor layer or when a large diameter substrate is used, the diameter of the platform 27 is inevitably increased and hence the size of the device.
Thus, a compact MOCVD device that has a capability of controlling the temperature of the source gas flowing through the reaction chamber along the substrate, is desired for the uniform quality of the semiconductor layer.